Inkjet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. It is used widely for commercial and business applications for printing on various substrates from paper to cable marking or wide format vinyl sheeting and across markets ranging from industrial labelling to short-run printing to desktop document and pictorial imaging.
There are various methods which may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired image. In one process, known as continuous inkjet, ink is supplied under pressure through orifices that produce jets of ink, which break up into a continuous stream of droplets which may be of different sizes. The droplets are subsequently sorted such that some droplets form the image whereas others are caught and recirculated. For example, droplets can be selectively charged as a means of sorting or their size can be selectively varied to allow them to be sorted by selective deflection using a stream of air. The droplets that have been caught can then be recycled from the catcher and redispersed within the bulk ink.
In another process, known as drop-on-demand inkjet, individual ink droplets are projected as needed onto the image-recording element to form the desired image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation.
The inks used in the various inkjet printers can be classified as either dye-based or pigment-based. A dye is a colorant, which is dissolved in the carrier medium. A pigment is a colorant that is insoluble in the carrier medium, but is dispersed or suspended in the form of small particles, often stabilized against flocculation and settling by the use of dispersing agents. The carrier medium can be a liquid or a solid at room temperature in either case. Commonly used carrier media include water, organic solvents such as alcohols, ketones or hydrocarbons, as well as mixtures of water and organic co-solvents, such as alcohols, esters and ketones.
An important characteristic of inkjet printing is the need to control the ink on the surface of the substrate onto which it is deposited. In the case of common inkjet recording elements, an important factor in achieving this is the absorption of significant portions of the ink, particularly the carrier medium, into some part of the substrate structure. As a consequence, the printed image can appear to be dry immediately after printing and the absorbed liquid can evaporate later. This allows organic solvents and co-solvents with low boiling points to be usefully incorporated into ink formulations, particularly for drop-on-demand inkjet printing.
However, liquid absorption does not occur when printing onto impermeable substrates and in this case either a very fast drying process is applied, much more volatile organic solvents are used as a major component of the carrier medium, or the ink droplets undergo some kind of phase-change on the substrate. All of these practices have significant disadvantages. For example, many impermeable substrates are heat-sensitive, many volatile organic compounds raise concerns about health and safety and phase-change inks produce significantly thicker printed layers because most of the ink droplet is solidified.
One of the advantages of inkjet printing is that it is a non-contact method and can be used to print onto a wide range of surface topography. However, the nature of the surface, particularly its surface energy, can still present difficulties. The surface energy quantifies the disruption of chemical bonds that occurs when a surface is created. It is the interaction between the forces of cohesion and the forces of adhesion which determines whether or not wetting occurs. If complete wetting does not occur, then a bead of liquid will form with a contact angle which is a function of the surface energies of the system.
Successful printing is normally achieved by applying inks with a surface tension lower then the surface energy of the surface. Unfortunately water has a very high surface tension, which makes it particularly difficult to apply satisfactorily as droplets onto low energy, impermeable surfaces.
Health and environmental concerns dictate a need for an ink with a significantly reduced, for example, not more then 10%, volatile organic solvent content. Patent Application Publication No. US 2003/0073758 describes an ink suitable for printing on flexible substrates wherein a small part of the volatile solvent has been replaced by water, but the major part of the ink still consists of a volatile organic solvent, such as ethanol or methylethylketone. Unfortunately, many aqueous inks are either not capable of sufficiently wetting the substrate or do not dry quickly enough at the speeds used in inkjet printing, especially in continuous inkjet printing wherein the time between successive drops is very significantly shorter than for drop-on demand inkjet systems.
Radiation-curable water-based inks have also been described for use in an inkjet process (patent application WO 99/07796), but often the resulting printed layers can be thick and may lack desirable physical properties, for example, sufficient flexibility. Such inks are usually high in viscosity, which makes the jetting process difficult, and it is often necessary to minimise viscosity by heating the ink significantly prior to printing. In addition, the curing process, which can involve complex equipment and considerable energy consumption, can in some cases be quenched by oxygen and require special measures, such as ‘blanketing’ under nitrogen. Moreover the printed image may be insufficiently hardened, or curing may continue after the initial curing time under the radiation source. Frequently the substrate needs to be treated to enable adequate adhesion of the cured ink to the substrate.
Other inks that work on a similar basis, often referred to collectively as ‘phase change’ inks, such as hot-melt inks, suffer from similar disadvantages. Hot-melt inks incorporate waxes which are solid at room temperature and melt when heated to decrease viscosity. Ink which is heated and melted in a printhead is ejected and deposited onto a substrate, usually paper, where it solidifies on cooling to form the printed images. This method avoids the use of volatile solvents but is, however, non-aqueous and one problem is the build-up of wax on the image where multiple droplets are placed to form a composite colour.
Another alternative is to use air-drying cross-linking polymers in a water-based ink (patent application WO 01/36547). However, the air-drying polymer can also harden around the orifices of a printhead, or on the recirculation catcher of a continuous inkjet printer, causing the same problems of poor image quality and productivity as mentioned hereinbefore for solvent-based inks. These polymers are extremely difficult to remove once they have cross-linked on the recirculation catcher, since eventually the hardened deposits block the catcher preventing further recirculation of non-printed drops, making them unsuitable for continuous inkjet printing.
U.S. Pat. No. 5,462,591 describes an aqueous phase change ink composition for use with a drop-on-demand inkjet printer and a porous substrate to reduce ‘feathering’ thereon, which comprises a water-soluble polyoxyalkylene or cellulose derivative that exhibits thermo-inversion properties, such that the water solubility decreases as the solution temperature increases. At a critical concentration, hyperthermogelation occurs on undergoing a phase transition to form stable gels on contact with a substrate. Typically on jetting an ink drop onto paper the drop will lose water by penetration therein and by evaporation, increasing the concentration thereby resulting in a rapid increase in viscosity. Alternatively the drop can be jetted onto a substrate that is above the thermo-inversion point, whereby contact with the warm substrate will instantly gel the drop.
JP06248208 discloses an ink composition comprising a monohydric alcohol and at least one water-soluble polymeric compound, wherein there is interaction between the polymer and the cellulose in the paper substrate. The composition disclosed therein is also for use with drop-on-demand inkjet printers to prevent feathering of ink drops when jetted onto the porous paper substrates, the aqueous ink in this case being inhibited from penetrating into the paper by molecular interactions.
However neither of the compositions disclosed in U.S. Pat. No. 5,462,591 and JP06248208 would be suitable for use under the high shear conditions employed during continuous inkjet printing, since polymer stringing could occur and drop ‘break-off’ would become unreliable.
US 2006/0128830 discloses that another means of viscosifying the ink when it has reached the substrate surface is to have a two-component ink wherein the components are mixed together in the ink but react on heating the substrate. However, the inks are dependent on significant heating (above 70° C.) of the substrate to achieve the required viscosity for droplet immobilisation, which restricts the type of substrates which can be printed by this method.
JP1999 236523 relates to inkjet printing on plain paper in which drying is assisted by the inclusion of volatile organic compounds or heating the substrate to around 80° C. It is emphasised therein that the substrate temperature has to be higher than the printhead temperature as swelling on the, generally porous, substrate has to be avoided. There is no mention therein of thermally responsive particles: the resins used may have thermally-switchable properties but the thermosensitive polymers are polyvinyl methylethers.
JP-05 148442 discloses pH-switchable particles in association with various colorants which swell on increase of pH. The ink has a pH above 6 to print on plain paper which relies on protons from the paper surface to gel the ink with the aim of reducing blotting of the ink droplets. Volatile water-soluble wetting agents are incorporated in the ink to lower surface tension and speed drying.
Pigment can be incorporated into an ink by encapsulating it in a polymer. U.S. Pat. Nos. 6,866,707 and 6,869,470 describe inks prepared from pigments coated with polymer resin which could then form a self-dispersed pigment via micro encapsulation. However, although stimulus-responsive materials, such as N-isopropylacrylamide, could be used to encapsulate pigment, the patents do not teach the use of these materials to enable a lower viscosity of ink in the printhead versus that of the substrate so that it could be used to print on a wide variety of substrates. Indeed the invention described in U.S. Pat. No. 6,866,707 is specifically for printing on plain paper.